1. Field of the Invention
The present invention relates to mass spectrometry. In particular, the present invention relates to an ion analyzer cell for use in ion cyclotron resonance mass spectrometry.
2. Discussion of Background
Ion cyclotron resonance mass spectroscopy (ICR-MS) is a sensitive technique for detecting gaseous ions. ICR-MS is based on the well known phenomenon that the motion of ions in a static magnetic field is constrained to a circular orbit in the plane perpendicular to the direction of the field. Ion motion is unrestrained in directions parallel to the field. The frequency of the circular motion is given by .omega..sub.c =(q/m)B, where q is the charge of the ions, m their mass, and B the magnetic field strength. If the field strength is known, the mass-to-charge ratio of the ions can be determined from measurements of the resonance frequency .omega..sub.c.
If a group of ions is subjected to a magnetic field and an oscillating electric field perpendicular to the magnetic field, those ions having an orbital frequency in resonance with the frequency of the electric field will absorb energy from the electric field and accelerate to a larger orbit; those with different frequencies will not. This behavior is used to distinguish the resonant ions from the non-resonant ions. The mass spectrum of a sample can be obtained by varying the frequency of the electric field, the field strength of the magnetic field, or both, so as to bring ions of differing mass-to-charge ratios into resonance with the electric field.
Several types of ICR-MS are available. For example, a gaseous sample may be bombarded with electrons to generate ions. The ions are directed through a region where they are subjected to mutually perpendicular static magnetic and oscillating electric fields. The ions ultimately strike a collector plate, and the resulting ion current is measured and recorded. In another type of ICR-MS, ions having a resonant frequency equal to the frequency of the oscillating electric field are accelerated and the resultant power absorbed from the electric field is measured. The measured power is related only to the resonant ions.
A disadvantage of these systems is that only a single frequency and a single mass-to-charge ratio can be detected at any given time. To obtain a mass-to-charge ratio spectrum, the magnetic field strength, the frequency of the electric field, or both, must be varied in step-wise fashion and the measurement repeated for each step. Fourier transform ion cyclotron resonance (FT-ICR) spectrometers such as that described by Comisarow, et al. (U.S. Pat. No. 3,937,955) provide faster data acquisition and detection efficiency than conventional ICR-MS systems. In an FT-ICR, ions are formed within an ion analyzer cell positioned in a homogeneous magnetic field. The ions are excited with a broad-band oscillating electric field pulse and their cyclotron motion is detected by amplification of the signal they induce in a set of receiver plates. Fourier transformation of the recorded signal provides a complete mass spectrum.
Ions for ICR-MS spectrometry are generated by a variety of techniques, including electron, ion, or laser beams directed at the sample to ionize it. The ions are held in an ion cyclotron resonance cell (analyzer cell; ICR cell) for analysis. The ICR cell is positioned in a vacuum chamber in a high strength, homogeneous magnetic field. The magnetic field prevents the ions from escaping in a direction perpendicular to the field, and a low voltage (trapping voltage) is applied to the end plates (trapping plates) of the cell to prevent the ions from escaping in the direction parallel to the field. In addition to the end trapping plates, ICR cells have paired side plates serving as excitation electrodes and detector electrodes. Grounded screens may be provided within the cell, just inside the end trapping plates, to reduce the electrostatic field in the cell resulting from the application of a potential to the end trapping plates (Marshall, et al., U.S. Pat. No. 4,931,640). ICR cells are available in several different geometries, including cubic, cylindrical, orthorhombic, hyperbolic, and multi-sectional cells.
For optimum performance, ICR-MS systems--including FT-ICR systems--must be operated at high magnetic field strengths and low pressures. Both the mass resolution and sensitivity of the system degrade seriously if the pressure in the ICR cell is higher than about 1.times.10.sup.-6 torr. In most ICR-MS systems, the samples to be analyzed are introduced to the ICR cell where they are ionized by any of variety of techniques, including electron impact, laser desorption, and so forth. High speed pumping systems are needed to maintain high vacuum conditions at the analyzer cell. The geometry of available high strength, superconducting magnets severely restricts access to the ICR cell and consequently the placement of ion generation devices. Typical vacuum chambers inside a superconducting magnet are on the order of 4"-5" in diameter. Collisional damping of the signal, resulting from sample ionization at moderate pressures (10.sup.-8 -10.sup.-5 torr) and analysis in the same cell, reduces the mass resolution and sensitivity of the system. In addition, an ICR cell can contain only a limited number of ions before their space charge seriously degrades the performance of the cell.
These problems are addressed by a multi-sectional ICR cell, wherein samples are introduced and ionized in one section, and analysis is performed in another section. Ions migrate between sections through a conductance limit plate that allows the maintenance of a pressure differential between the cell sections (Littlejohn, et al., U.S. Pat. No. 4,581,533). This arrangement reduces collisional damping in the analysis cell, but does not fully address the sample handling problems resulting from ion production within the bore of the magnet.
Another approach involves the use of external ion sources, where the ions are generated outside the magnetic field and transferred to the ICR cell. For example, McIver, Jr. (U.S. Pat. No. 4,545,235) uses a quadruple mass filter to inject ions from an external source into an ICR-MS system. The ions are injected parallel to the applied magnetic field and are trapped in the ICR cell for relatively long time periods during which analyses are performed. Electrostatic lenses or pulsed high voltage lenses are used to direct ions from an external source to an ICR cell, such as in the apparatus described by Ghaderi, et al. (U.S. Pat. No. 4,739,165). These arrangements are complex and increase the size and cost of the overall system. Without careful shielding, electrical interference from a quadrupole or electrostatic lens can affect the detection circuitry of the ICR cell.
FT-ICR is the highest resolution mass spectrometry technique currently available. When used with a laser for inducing fluorescence of the ions, FT-ICR is an ideal technique for identification and characterization of small samples, including but not limited to analysis of trace contaminants in micro-electronic devices. However, laser induced fluorescence (LIF) is difficult to carry out, due to low ion concentrations, interference from stray light or scattered light from the irradiating beam, the small amounts of light produced, and the operating constraints imposed by ion sources. Similar problems are encountered in applications of other mass spectrometric techniques, including collision-activated dissociation and laser photodissociation.